Three-dimensional printing, developed at the Massachusetts Institute of Technology in the early 1990s is a versatile and useful process for creating a wide variety of structures for engineers and designers. See, U.S. Pat. No. 5,204,055. All of the references cited herein are incorporated by reference.
The three-dimensional printing method, as practiced by Z Corporation in their line of products, provides a supply of dry, granular material (hereinafter referred to as “powder”) in a sequence of thin layers from one source; and a printed pattern of a liquid reaction medium (hereinafter referred to as “binder”) from another source. The powder is formed into a flat layer by a mechanism, preferably a counter-rotating roller. The binder is dispensed most usually by an inkjet printhead. Chemical reactants incorporated in the powder are stable in the dry mixture until the liquid binder is added locally to a particular volume of the powder. The liquid binder provides a reaction medium and, optionally, additional reactants to the system.
In the operation of a three-dimensional printer, a layer of dry powder is first spread over a pre-existing surface. This layer generally ranges from about 50 microns to about 250 microns in thickness. After the layer is spread, an ink-jet printhead deposits a patterned dose of liquid binder over the surface. The pattern coincides with a cross-section of a desired object. The liquid binder infiltrates the powder and initiates a chemical reaction that results in the cementation, or bonding, of the granular material into a solid structure. The structure becomes bonded to any previously existing structure by migration of the liquid binder through the top layer of powder to meet bonded material in the layer immediately below. Following the formation of a given layer, the spreading mechanism deposits a further layer of powder over and in contact with the previous one, a subsequent pattern of liquid binder is dispensed, and the process is repeated until a finished article is constructed as the union of many layers bonded vertically.
The granular powdered material forms the bulk of the finished article. Portions of the liquid component may be incorporated chemically into the finished article, but it generally comprises a very small fraction of the total mass. The physical properties of the finished article are primarily determined by the nature of the granular feedstock. Refractory materials have been selected as the powder component in the instant invention to provide tolerance to high temperatures required during casting of ferrous alloys.
Dry powder remains in regions surrounding the regions moistened with liquid binder. Although it is technically not solid, these portions of loose, unbonded powder comprise a temporary fixture for the solidified material during intermediate stages of construction. The friction characteristics of the loose powder must be chosen to ensure a proper degree of mechanical support, otherwise the forces applied during layer definition can cause undesired movement and distortion.
One of the earliest applications for three-dimensional printing was the direct production of ceramic molds for metal casting, especially for high-temperature metals used in the aerospace industry. See, Bredt, J., “Binder Stability and Powder\Binder Interaction in Three Dimensional Printing,” Ph.D. dissertation M. E. 1995, M.I.T., Cambridge, Mass. Ingo Ederer developed both a three-dimensional printer and a materials system for creating resin-bonded sand molds. See, European Patent No. 1,268,165. These molds were compatible with ferrous casting techniques. The ZPrinter® product line developed and sold by Z Corporation of Burlington, Mass. has proven to be a versatile and reliable platform for a variety of rapid prototyping techniques and Z Corporation developed a materials set for building molds for non-ferrous metals, and a second system for ferrous metal casting. See, U.S. Pat. No. 7,087,109. However, the ferrous casting systems have not yet been marketed successfully.
This prior art suffered from limitations that prevented them from becoming widely used. In the case of the early development of three-dimensional printing at MIT, the MIT process required the printing engine to dispense a heavily loaded suspension of colloidal silica, and suffered from severe printhead reliability problems as a result. The process was commercialized by Soligen Tech in the 1990's but it was not successful due to hardware maintenance problems stemming from handling the colloidal silica. Ingo Ederer's system used a liquid catalyst solution of either methanesulfonic acid (MSA) or sulfuric acid, and required specialized printing hardware to utilize. In the Z Corporation case for casting non-ferrous metals, the casting material was based on gypsum and is therefore unsuitable for use at the higher temperatures required for casting iron and steel.
In the case of the Z Corporation materials system for ferrous casting all three of the previous limitations were overcome, but the material formulation required the addition of a small, but significant, amount of organic adhesive in order to achieve the necessary green strength in its as-printed state. This organic material was a source of pernicious smoke during the subsequent baking and casting steps, and it severely limited the market potential of the product.